With the diminishing supply of crude oil, the use of renewable energy sources is becoming increasingly important as a feedstock for production of hydrocarbon compounds. Plants and animal biomass are being used to produce liquid and gaseous hydrocarbon compounds. One of the advantages of using biomass is that the CO2 balance is more favourable as compared with the conventional hydrocarbon feedstock.
One of the existing processes for the conversion of biomass includes the steps of pyrolysing the lignocellulosic material derived from plants to obtain a pyrolysis oil, and upgrading the pyrolysis oil to obtain chemicals and fuel products.
The pyrolysis oil obtained from the pyrolysis of a feed containing lignocellulose is or contains a mixture of oxygenated compounds, formed during the decomposition of lignin and cellulose (possibly including hemicellulose) in the biomass, and water generated during the process and from the initial moisture content of the biomass. The oxygen and water content in the pyrolysis oil is believed to cause a significant reduction in the heating value of the pyrolysis oil, for example compared to conventional oil. Further, the complex chemical nature and high viscosity adds to the difficulty of processing this pyrolysis oil in standard refinery units. Hence, several processes for upgrading the pyrolysis oil have been proposed in the literature. Examples of these processes include Hydrodeoxygenation (HDO) under high hydrogen pressures, Catalytic Cracking and High Pressure Thermal Treatment (HPTT). These upgrading processes for the pyrolysis oil may involve, for instance, removal of the oxygen (usually >95%), decarboxylation, viscosity reduction, sulfur removal, nitrogen removal, and the like.
Usually the existing Hydro-deoxygenation (HDO) processes require high pressures of hydrogen, for instance, in the range of 250 bar to 350 bar(a), for the removal of oxygen from the pyrolysis oil in the form of water. Some of these processes also suggest a multi-step hydrodeoxygenation to achieve significant (˜95%) oxygen removal. These processes entail very high hydrogen consumption, which makes them uneconomical and difficult.
The most common process for conversion of biomass to hydrocarbon products is by catalytic cracking.
US-A-2008/00538705 describes a method of conversion of pyrolysis oils to useful hydrocarbon products by the process of hydrotreatment followed by hydrocracking in the presence of a catalyst. However, the catalytic cracking process results in production of coke and its subsequent deposition on the catalyst. Further the catalytic cracking process also has limitations such as reactor problems and additional expenses.
US-A-2009/0253948 describes a method of conversion of pyrolysis oils to useful hydrocarbon products by the process of partial hydrotreatment followed by full hydrotreating in the presence of a hydrocracking catalyst. However, the catalysts proposed in this process may decay under the reaction conditions, specifically under the conditions if in the hydrodeoxygenation step a large amount of water is present. This may lead in the best situation to catalyst inactivation through leaching of active components, but may also lead to weakening of the catalyst particles and/or clogging of the catalyst pores that can lead to pressure build-up in the reactor.
In his master thesis titled “Hydrotreating of pyrolysis oil” dated 7 Aug. 2006, Evert Leijenhorst describes the feasibility to partly deoxygenate pyrolysis oil. He mentions that low severity hydrotreating involves partial hydrodeoxygenation, minimal hydrocracking and effective hydrogenation. He further indicates that low severity hydrotreating is an attempt to upgrade the pyrolysis oil to produce turbine fuels for electricity generation or as a first step towards co-feeding in a conventional petroleum refinery (see section 2.4). The maser thesis mentions that the pyrolysis oil may be obtained from wood (see section 1.2). The master thesis further includes several experiments including a two-stage hydrotreating of pyrolysis oil (sections 5.5) leading to products comprising an oil phase having an oxygen content in the range from 19.7 wt % to 27.0 wt %.
Marsman et al. in their article titled “Identification and classification of components in flash pyrolysis oil and hydrodeoxygenated oils by two-dimensional gas chromatography and time-of-flight mass spectroscopy, J. Chromatogr. A 1188 (2008) pages 17-25, made available on the internet on 14 Feb. 2008, describe hydrodeoxygenation of pyrolysis oil produced from beech flakes at a pressure of 25 MPA and a temperature of 573K (about 300°) over a Palladium on carbon catalyst and a Ruthenium on carbon catalyst.
Ardiyanti et al. in their article titled “Process-product studies on pyrolysis oil upgrading by hydrotreatment with Ru/C catalysts, first presented at the AICHE 2009 spring meeting in April 2009, mentioned that pyrolysis oil is not suitable for the purpose of co-feeding into existing refineries, either in hydrotreating or FCC units, because the oil is not miscible with hydrocarbon feedstocks and shows a high tendency for coking, leading to blockage of feeding lines and reactors. As an alternative a mild hydrotreating process is suggested. The article describes several experiments. In experiment 4, a pyrolysis oil, obtained by fast pyrolysis of forest residue, was hydrotreated using Ru/C as a hydrotreating catalyst; a fixed hydrogen pressure of 200 bar; and a varying temperatures in the range from 175° C. to 350° C. The product oil obtained at these conditions, having an oxygen content of 17.6 wt %, was subjected to a second hydrotreating procedure, using Ru/C as a hydrotreating catalyst; a fixed hydrogen pressure of 200 bar; and a varying temperatures in the range from 350° C. to 400° C. Two phases were formed, viz. a black oil floating on top of a clear water layer. The oxygen content of the oil was reduced to respectively 12.3 wt % and 11.5 wt %.
Hence, while some processes for upgrading the pyrolysis oil to produce hydrocarbon products have been described, there is a need for an improvement in the processes for conversion of pyrolysis oils to useful hydrocarbon products.